Preclinically, the effect of a combination of anti-cancer drugs can be studied in vitro on cell lines or in vivo with different tumor models. Typically, anti-cancer drugs that have different mechanisms of killing, i.e. have different targets in the cell, are combined. In such experimental systems, it was observed that two anti-cancer drugs with independent targets (mutually exclusive drugs) either behave in an additive, synergistic, or antagonistic manner. Chou and Talalay (Adv. Enzyme Regul. 1984, 22:27-55) developed a mathematical method that could accurately describe the experimental findings in a qualitative and quantitative manner. For mutually exclusive drugs, they showed that the generalized isobol equation applies for any degree of effect (see page 52 in Chou and Talalay). An isobol or isobologram is the graphic representation of all dose combinations of two drugs that have the same degree of effect, for example combinations of two cytotoxic drugs will affect the same degree of cell kill, such as 20% or 50% of cell kill. The equation is valid for any degree of effect and the graphic representation will have the same shape (page 54, line 1, in Chou and Talalay), which is presented in FIG. 11 D (page 5 in Chou and Talalay). In isobolograms, a straight line indicates additive effects, a concave curve (curve below the straight line) represents synergistic effects, and a convex curve (curve above the straight line) represents antagonistic effects. These curves also show that a combination of two mutually exclusive drugs will show the same type of effect over the whole concentration range, either the combination is additive, synergistic, or antagonistic. Most drug combinations show an additive effect. In some instances however, the combinations show less or more than an additive effect. These combinations are called antagonistic or synergistic, respectively. Antagonistic or synergistic effects are unpredictable, and are unexpected experimental findings. A combination manifests therapeutic synergy if it is therapeutically superior to one or other of the constituents used at its optimum dose. See T. H. Corbett et al., Cancer Treatment Reports, 66, 1187 (1982). Tallarida R J (J Pharmacol Exp Ther. 2001 September; 298 (3):865-72) also notes “Two drugs that produce overtly similar effects will sometimes produce exaggerated or diminished effects when used concurrently. A quantitative assessment is necessary to distinguish these cases from simply additive action”.
That the unpredictability of antagonistic or synergistic effects is well known to one of skill in the art is demonstrated in several other studies, such as, by Knight et al. See BMC Cancer 2004, 4:83. In this study, the authors measured the activity of gefitinib (also known as Iressa) alone or in combination with different cytotoxic drugs (cisplatin, gemcitabine, oxaliplatin and treosulfan) against a variety of solid tumors including breast, colorectal, esophageal and ovarian cancer, carcinoma of unknown primary site, cutaneous and uveal melanoma, non-small cell lung cancer (NSCLC) and sarcoma.
They discovered that there was heterogeneity in the degree of tumor growth inhibition (TGI) observed when tumors were tested against single agent gefitinib. In 7% (6/86) of tumors considerable inhibition of tumor growth was observed, but most showed a more modest response resulting in a low degree of TGI. Interestingly, gefitinib had both positive and negative effects when used in combination with different cytotoxic drugs. In 59% (45/76) of tumors tested, the addition of gefitinib appeared to potentiate the effect of the cytotoxic agent or combination (of these, 11% (5/45) had a >50% decrease in their IndexSUM). In 38% of tumors (29/76), the TGI was decreased when the combination of gefitinib+cytotoxic drug was used in comparison to the cytotoxic drug alone. In the remaining 3% (2/76) there was no change observed.
The authors conclude that gefitinib in combination with different cytotoxic agents (cisplatin; gemcitabine; oxaliplatin; treosulfan and treosulfan+gemcitabine) is a double-edged sword: their effect on growth rate may make some tumors more resistant to concomitant cytotoxic chemotherapy, while their effect on cytokine-mediated cell survival (anti-apoptotic) mechanisms may potentiate sensitivity to the same drugs in tumors from other individuals. See conclusion on page 7; see also FIG. 3. Knight et al., BMC Cancer 2004, 4:83.
Thus, this study proves that two compounds, which are known to be useful for the same purpose, are combined for that purpose, may not necessary perform the same purpose.
Finding highly efficacious combinations, i.e., synergistic mixtures, of active agents is challenging however. Serendipity is not a valid route as the number of potential combinations of agents is staggeringly large. For example, there are trillions of possible 5 fold combinations of even a relatively small palette of 5000 potential agents. The other normal discovery strategy of deducing potential combinations from knowledge of mechanism is also limited in its potential because many biological end points of living organisms are affected by multiple pathways. These pathways are often not known, and even when they are, the ways in which the pathways interact to produce the biological end effect are often unknown.
Previously, we demonstrated synergistic combination of a maytansinoid immunoconjugate comprising a maytansinoid compound linked to a monoclonal antibody with that of a taxane compound, an epothilone compound, a platinum compound, an epipodophyllotoxin compound and a camptothecin compound.
Synergistic uses of combination of drugs even if previously demonstrated do not obviate the need to look for new synergistic combinations because synergistic effects are unpredictable and because these are unexpected experimental findings. For example, in treatment of autoimmune deficiency syndrome (AIDS), which involved highly active anti-retroviral therapy (HAART), it was believed that cocktail of inhibitors of HIV-1 viral reverse transcriptase (RT) and the viral protease (PR), exhibit synergistic inhibition of viral replication. Later on, intriguingly, synergy was also observed within two classes of RT inhibitors—that is, the nucleoside RT inhibitors (NRTIs) showed synergy with the nonnucleoside RT inhibitors (NNRTIs) in the absence of PR inhibitors. For example, NRTI, AZT (zidovudine) and the NNRTI, nevirapin exhibit synergy when given in combination (Basavapathruni A et al., J. Biol. Chem., Vol. 279, Issue 8, 6221-6224, Feb. 20, 2004). Thus, there is still a need for finding drug combinations that show synergism and can be effectively used for the treatment and prevention of debilitating diseases such as cancer.